Editing the Web: Detecting the Lost Update Problem Using Unreserved Checkout

Avoiding the lost update problem has been a notorious challenge when editing documents remotely on the Web using HTTP/1.0. While WebDAV provides an extended set of services for editing the Web, HTTP/1.1 provides a minimal set of hooks for avoiding the lost problem by detecting when versions have changed so that changes aren't lost in the editing process. While simple, these hooks are fundamental to editing the Web using HTTP/1.1 and are needed in Webdav as well. This Note explains a) how to use HTTP/1.1 to detect the lost update problem using preconditions and strong etags and b) how to avoid problems with HTTP/1.0 clients that do not know about these features but only use plain HTTP PUT requests. Neither a) nor b) requires any changes to HTTP/1.1, but can be achieved using existing features. The mechanism has been implemented in Web Commander and Amaya (both using libwww), and Jigsaw — all W3C Open Source software freely available to all interested parties. Detection is only one of several ways to avoid the lost update problem and this document discusses the pros and cons of various other mechanisms including exclusive locks and immutable revisions.